The field of heat exchanger designs is replete with applications of fins to improve the heat transfer. Typically, this is heat transfer by forced convection mechanism. The heat transfer by forced convection takes place between a solid surface and fluid in motion, which may be gas or liquid, and it comprises the combined effects of conduction and fluid flow. This type of heat transfer occurs in most of the conventional heating systems, either hot water or electric and industrial heat exchangers.
In the cracking process of a paraffin, such as, ethane or naphtha, the feed flows through a furnace coil (pipe) that is heated up to 1100° C., inside the radiant section of a cracking furnace. At these temperatures, the feed undergoes a number of reactions, including a free radical decomposition (cracking), reformation of a new unsaturated product and the coproduction of hydrogen. These reactions occur over a very short period of time that corresponds to the feed residence time in a coil.
The interior of the radiant section of the furnace, is lined with heat absorbing/radiating refractory, and is heated typically by gas fired burners. The heat transfer within the furnace, between flame, combustion gases, refractory and the process coils is mostly by radiation, and also by (forced) convection.
A need exists for improvements to the efficiency of cracking furnaces as this reduces process costs and greenhouse gas emissions. There have been two main approaches to improving efficiency; improving heat transfer to the furnace coils, i.e., from flame, combustion gases and refractory walls to the external surface of a process coil, and improving heat transfer within the coil, i.e., from the coil walls into the feed flowing inside the coil.
One of the methods representing the second approach is the addition of internal fins to the inner walls of the furnace coil, to promote the “swirling” or mixing of the feed within the coil. This improves the convective heat transfer from the coil walls to the feed as the turbulence of the feed flow is increased and the heat transferring surface of the hot inner wall of the coil is increased as well.
While some believe that a tube with internal helicoidal fins performs better than a tube with internal longitudinal fins and that the results for “a tube with internal helicoidal fins are in excellent agreement with industrial observations.” However, Applicants are not aware of any experimental data to support this belief. In addition Applicants are not aware of any comparisons made to the performance of a bare tube, with no internal ribs or fins. Tubes for use in the convection section of a cracking furnace, where the pins are on the surface of the downward face of the tubes in the convection section of the furnace are known. The pins are tightly packed and dimension for the length of the pins are unknown.
Elongated plates pivotably mounted on at least one horizontal tube in a vertical row that is one row removed from the wall are also known. When the plate is pivoted down, it prevents channeling of the hot gases through the convection section of the furnace (flue).
Putting “studs” (“pins”) on the external and internal surfaces of pyrolysis coils used in the radiant section of a cracker is known. The pins are not arranged in longitudinal rows and have a length from 0.5 to 0.75 inches.
Pipes for chemical reactions, such as furnace tubes having internal grooves are also known.
Furnace tubes having studs 2 inches long and 0.5 cm in diameter placed in longitudinal arrays on the side of the furnace tube facing the refractory wall are also known.
A need exists for an enhanced heat transfer, comparable to that of a fin, while reducing the stress on the tube or pipe.